In general in an induction machine used as a generator, the frequency of the alternating electrical current generated varies with rotor speed. In applications where the speed at which the rotor is driven can vary, such as with varying wind speed in a wind power generator or turbine, or where power is generated from a varying speed water flow, the output power frequency will vary unless this is addressed in the generator or turbine design.
The use of small generators (sometimes called micro-turbines) driven by wind or hydro power is increasing, for power generation at a domestic or small agricultural or industrial scale for example, but uptake is subject to factors including the initial installation cost to the home or small enterprise.
A household or small enterprise utilising local scale power generation often has the option of feeding excess power back into the main electric power distribution grid. Power fed into the grid must be at mains frequency.
AC voltage with variable frequency can be rectified to DC and then inverted back to constant frequency AC voltage to produce AC power. This however requires an additional power electronics stage at the output of a generator, which increases complexity and/or cost. Alternatively a constant speed mechanical drive can be used but at the expense of not being able to extract maximum possible energy that is available. Alternatively again a doubly fed induction machine can be used. An exciting frequency which varies with rotor speed is fed to the rotor in such a way that a constant frequency of AC is output by the machine. This also requires more sophisticated electronics, for providing the varying frequency current to the rotor.